1. Field of the Invention
The present invention relates to an object detector for an optical instrument that is suitable for detecting an object of image pickup in the case of the automatic focusing of the optical instrument, such as a camera, etc.
2. Discussion of the Related Art
As an automatic focusing system for an optical instrument such as a camera, etc., a so-called passive system for catching the image of an object or subject bathed in natural light or illumination light to adjust the focus of an image-pickup lens corresponding to the condition thereof has particularly attracted public attention in recent years. The passive system attains a higher focusing accuracy and consumes less electric power than a so-called active system for projecting pulses of infrared light or the like to an object to adjust the focus of an image-pickup lens according to the distance to the object detected on the basis of the turnaround time of the pulses. The practical use and spread of the passive system has been developed rapidly. As is well-known in the art, the passive systems may be roughly classified into two types: (1) a triangular surveying system for detecting the distance to an object on the basis of so-called outside light without use of the image-pickup lens, and (2) a TTL (Through The Lens) system for detecting the difference of the focusing condition on the basis of inside light passing through the image-pickup lens.
The basic principles of the outside light triangular surveying system will now be described in brief with reference to FIG. 8. In FIG. 8, a pair of small lenses 11 and 12 are disposed in an optical instrument as an optical means for catching the image of an object A. The pair of small lenses 11 and 12 are separated by a base line length b, receiving light from the object A through different optical paths L1 and L2. Images I1 and I2 of the object A are formed respectively in the illustrated positions on image sensors 21 and 22 placed at the focal lengths f of lenses 11 and 12. When the object A is infinitely distinct from the optical instrument, the positions of the images I1 and I2 are respectively at the intersection of an optical axis Lc, passing through the centers of the lenses 11 and 12, and image sensors 21 and 22. When the object A approaches the optical instrument, however, the positions of images I1 and I2 are respectively shifted by the distances designated by s1 and s2 in the drawing in reverse directions from such original positions.
Assuming that the distance from the lenses 11 and 12 to the object A is x and that the optical axis passing through the object A separates the base line length b into b1 and b2, the following expression should hold true because a right-angled triangle with sides x and the base line length part b1 is similar to a triangle with sides of focal length f and the shift amount s1. EQU b1/x=s1/f
The following expression should apply in the same manner as described above. EQU b2/x=s2/f
Assuming that the relation between s1 and s2 is expressed by s=s1+s2, the following expression results from the relation between b1 and b2 expressed as b=b1+b2. EQU b/x=s/f
Accordingly, if the sum s of the shift amounts s1 and s2 from the respective original positions of the images I1 and I2 is detected by any means, the distance x can be determined as follows. EQU x=bf/s
Because the images I1 and I2 of the object A are not points but always have some patterns, the shift amount s is detected by using, as patterns, image data groups obtained by collecting image data expressing the intensity of light applied to respective photosensors within the left and right image sensors 21 and 22. The number S of shifts required for adjusting the image patterns is measured while successively shifting the two image data groups one by one relative to the infinite distance reference points.
The shift amount s can be calculated by multiplying this number S of shifts by the pitch of arrangement of the photosensors within the image sensors. Accordingly, the focus of the optical instrument can be adjusted to the object A by calculating the distance x on the basis of the shift amount s according to the aforementioned expression and adjusting the image-pickup lens to a focusing position corresponding to the distance x. In practical use, the position of the image-pickup lens of the optical instrument is adjusted by the number S of shifts to save the time required for calculating the shift amount s and the distance x.
The aforementioned image data may be digital data, for example, of 4-8 bits, so that in most cases the left and right image data groups do not perfectly accord with each other when the number S of shifts is measured. Therefore, in general, the number S of shifts representing a maximum correlation is detected by successively examining correlations between the two image data groups according to a suitable evaluation function while shifting the respective image data groups. Furthermore, if a shift value S as a decimal, instead of the number of S shifts as an integer, is calculated by using an interpolation technique in which the maximum correlation is detected by reference to evaluation function values and the surrounding values, the focusing accuracy in the optical instrument may be greatly improved.
Where the view field examined to detect the object is so wide that a plurality of objects enter into the field, narrowing of the field may be necessary for accurate focusing because the object to be detected may be undetermined. When the view field is narrowed, however, the picture thus obtained may be out of focus because the optical instrument 1 may detect an infinite background in the direction of the optical axis Lc if the instrument is aimed at a middle position between two objects A1 and A2 as shown in FIG. 9. This may occur, for example, where two persons are standing side by side.
To solve the disadvantage caused by a so-called center spoiling phenomenon such as this, without the necessity of widening the view field for detection of the object, a technique for detecting the distance to an object in an oblique direction with respect to the optical axis (see Japanese Patent Unexamined Publication Nos. Sho-60-15506 and Sho-61-120001) as proposed in the present invention can be utilized.
By using such an oblique-direction distance measuring technique, distances to an object can be detected in several directions (for example, three directions) including the direction of the optical axis Lc and oblique directions making an angle .alpha. with respect to the optical axis while the direction of the finder of the optical instrument is fixed as shown in FIG. 10. The optimum object to photograph can then be selected from the three detection results to adjust the focus of the optical instrument. In the conventional so-called three-point distance measuring technique, however, the inclination angle .alpha. is generally fixed because it is difficult to adjust. Accordingly, the technique is effective only in the case where the position of the object is aligned with the fixed oblique directions of view.